1. Field of Invention
The present invention relates to a surface light source device which is applicable to backlighting for liquid crystal display and other various devices for illumination or display.
2. Related Art
A known liquid crystal display has a liquid crystal display panel on the back side of which a surface light source device is disposed for backlighting. The surface light source device is provided with a light guide plate having a size and plane shape the same as those of the liquid crystal display panel, a primary light source disposed beside the light guide plate and a reflection plate disposed on the back side of the light guide plate, being called surface light source device of side light type.
Demand for thin-structured liquid crystal displays has been growing recent years. This brings a tendency that surface light source devices employ thin light guide plates. Such a tendency requires small light emitting sources. Small-sized light emitting sources (primary light sources) are used for supplying light to thin light guide plates. Light emitting diode (LED) is a typical small light source. FIGS. 4 and 5 illustrate surface light source device 100 employing LEDs 101 as light emitting source (primary light source) 101 in accordance with a prior art. LEDs 101 are disposed at a side face of light guide plate 102. Each LED denoted by reference numeral 101 is package-type LED, including a light emitting portion (light emitting element) 103.
Each light emitting portion (light emitting element) 103 is resin-sealed by resin package 104 so that light radiating face 103A is exposed from resin package 104. As shown in FIG. 4, the thickness of LED 101 including package 104 is set to that of light guide plate 102 at the side face (incidence side face) opposite to LED 101 for the sake of convenience in assembling. Accordingly, the length of light radiating face 103A along light guide plate thickness is smaller than the thickness of light guide plate 102 at the incidence side face.
Another prior art is disclosed in Tokkai-Hei 10-293202 (pp. 3 to 4, FIG. 1), also showing that the length of a light radiating face along light guide plate thickness is shorter than the thickness of a light guide plate in the vicinity of an incidence side face. Such light guide plates are subject to unevenness in brightness that appears on an emission face in the vicinity of an incidence side face. According to the above prior art, a plurality of grooves giving incident light diversified travelling directions are formed on the incidence side face for preventing this.
However, the brightness unevenness preventing art is apt to fail to reduce enough unevenness in brightness because LED is disposed so that its light radiating face is in contact with (or very close to) an incidence side face of an light guide plate. This situation is illustrated exemplarily in FIGS. 4 and 5.
There are upper and lower areas 105A, 105B on which light radiating face 103A does not abut since light radiation face 103A can not cover the thickness of light guide plate 102 at incidence side face 105 as described above. Primary light is not incident to areas 105A and 15B, providing a shade source which generates dark band 106 in light guide plate 102. This dark band 106 causes emission face 102A of light guide plate 102 to have stripe-like uneven brightness, which is observed as shown in FIG. 5. In other words, illumination light has a reduced quality.
A known light guide plate has a generally rectangular emission face (a major face) and a generally wedge-shaped cross section, being used frequently. The light guide plate has a back face (another major face opposite to the emission face) which is inclined as to get closer according to an increasing distance from an incidence side face. It is also known to form a plurality of prism projections running vertically to the incidence side face on the back face.
If such a light guide plate is used with a LED disposed as described above, incident light is reflected toward the emission face, after entering within the light guide plate, in a manner such that the light gets a restricted angular extension due to being effected by the prism projections formed on the back face. As a result, the foresaid dark band is not expanded, being seen clearly on the emission face. This tends to bring a conspicuous stripe-like uneven brightness.
As described above, a liquid crystal display employing surface light source device 100 involving dark band 106 is hardly able to provide a high-quality image on a liquid crystal display panel.